As a coordinate input apparatus for use in specifying a position on a screen of a display device or displaying lines such as characters and figures on the display device, various pointing devices such as mouse-type devices, pen-type devices, and track balls have been known.
Recently, pen-type devices (coordinate input pen) using ultrasonic waves have been proposed as pointing devices and have attracted attention. The coordinate input pen based on the ultrasonic wave method oscillates an ultrasonic wave by the coordinate input pen and calculates the position of the coordinate input pen on the basis of a propagation period from a time when an ultrasonic wave is oscillated to a time when a receiver fixed on a pad member (input face) such as a paper receives the ultrasonic wave, thereby inputting the input position of the coordinate input pen and manuscript writing in a computer. The foregoing technique allows manuscript input with ease. Further, this ultrasonic wave method allows input of exact input position of a pen and exact lines of the pen without providing a special board or a paper.
In a coordinate input apparatus adopting the foregoing coordinate input pen using ultrasonic waves, in order to accurately detect an input position of the coordinate input pen, it is necessary to maintain an output level of an ultrasonic wave from the coordinate input pen constant regardless of changes in stroke pressure applied on the coordinate input pen when an input instruction is given to the input face by the coordinate input pen. Japanese Unexamined Patent Publication No. 175580/1995 (Tokukaihei 7-175580; published on Jul. 14, 1995) discloses an example structure wherein an output level of an ultrasonic wave as emitted from the coordinate input pen can be maintained constant.
In the technique disclosed in Japanese Unexamined Patent Publication No. 175580/1995, a pressure sensor provided in the input pen detects a pressure (stroke pressure) applied on the input pen in a direction of the central axis of the frame of the input pen, and an operation control circuit controls a driving voltage control circuit to adjust a driving voltage for driving an oscillating section which oscillates an ultrasonic wave.
To be specific, in the coordinate input pen based on the ultrasonic wave method disclosed in Japanese Unexamined Patent Publication No. 175580/1995, in a case where the driving voltage for driving the oscillating section is constant, when a stroke pressure applied on the input pen is high, an output level of the ultrasonic wave increases. Therefore, the operation control circuit performs control so that the driving voltage drops as the stroke pressure increases.
For example, when the stroke pressure is low, the operation control circuit performs control so that the driving voltage increases. By driving the oscillating section with the increased driving voltage, it is possible to obtain a desired output level of an ultrasonic wave. Further, when the stroke pressure is high, the operation control circuit performs control so that the driving voltage drops. By driving the oscillating section with the dropped driving voltage, it is possible to obtain a desired output level of an ultrasonic wave.
Namely, in the technique disclosed in Japanese Unexamined Patent Publication No. 175580/1995, the driving voltage for driving the oscillating section is changed according to a change in the stroke pressure, so that the output level of the ultrasonic wave oscillated by the oscillating section is constant.
The foregoing conventional structure has the following problem. That is, to maintain the output level of the ultrasonic wave oscillated by the oscillating section constant, it is necessary to adjust a driving voltage for driving the oscillating section driving circuit according to changes in the stroke pressure applied on the coordinate input pen, which forces the driving circuit to have a complex structure.